![[H]ardOCP](/images/blank.gif){kind=small}
- Date:
- Thursday , May 19, 2005
- Author:
- Kyle Bennett
- Google +1
Xbox 360 GPU Features
More facts make their way to the surface concerning the Xbox 360 graphics processor, codenamed XENOS. This time ATI’s Vice President of Engineering chimes in on the unique technology that is powering the pixels behind Microsoft’s new gaming console.
About the Hardware
While we have tried our best to get better pictures of the Xbox 360 internals, we came up short. While folks at Microsoft and ATI will definitely not like to have the comparison made, there really is “just” a PC inside the Xbox 360...that happens to be a Mac. You can find a full table of Xbox 360 specifications here, but in the interest of speeding things up a bit, below is the short list covering video.
Xbox 360 Video System Performance & Specifications | |
Custom ATI Graphics Processor | *500 MHz *10 MB embedded DRAM *90nm process *48-way parallel floating-point dynamically-scheduled shader pipelines- Unified shader architecture |
Polygon Performance | *500 million triangles per second |
Pixel Fill Rate | *16 gigasamples per second fillrate using 4X MSAA |
Shader Performance | *48 billion shader operations per second |
Memory | *512 MB GDDR3 RAM *700 MHz DDR *Unified memory architecture |
Memory Bandwidth | *22.4 GB/s memory interface bus bandwidth *256 GB/s memory bandwidth to EDRAM *21.6 GB/s front-side bus |
HD Game Support | *All games supported at 16:9, 720p and 1080i, anti-aliasing *Standard definition and high definition video output supported |
GPU & Northbridge in One!
Many of you read that the Xbox 360 will have 512MB of GDDR3 RAM and that is 100% correct. Many of you may also be wondering exactly how the RAM works with the CPU and GPU. Once you learn that the Xbox 360 GPU also acts as the system’s memory controller, much like the Northbridge in an Intel PC, the picture becomes a bit clearer. ATI has been making and designing chipsets for a good while now that use GDDR3 RAM. Add to this that Joe Macri (go cart racing fiend extraordinaire), who was a pivotal factor in defining the GDDR3 RAM specification at JEDEC and is also a big fish at ATI, and it only makes sense that ATI could possibly put together one of the best GDDR3 memory controllers in the world. So while it might seem odd that the Xbox 360 Power PC processor is using “graphics” memory for its main system memory and a “GPU” as the “northbridge,” once you see the relationship between the three and the technology being used it is quite simple. Therefore, we have the 700MHz GDDR3 RAM acting as both system RAM and as GPU RAM, connected to the GPU via a traditional GDDR3 bus interface that can channel an amazing 25 Gigabytes per second of data.
Now between the GPU and the CPU, things get a bit fuzzier. And by “fuzzier,” I mean that they would not tell me much about it at all. The bus between the CPU and GPU was characterized as unique and proprietary. Mr. Feldstein did let on that the bus could shuttle up to 22 Gigabytes of data per second. Much like GDDR3, this would be a full duplex bus, or one that “goes both ways” at one time. Beyond that, not much was shared.
Viewing the Xbox 360 GPU as the “northbridge” should give you a better idea of how the Xbox works and answer some of the overall architecture questions. It is my own opinion that it is very likely that the CPU/GPU bus is very similar to the GPU/RAM bus as it was stressed to me by Mr. Feldstein that the CPU/RAM pathway was very free of any latency bottlenecks. The world may never know for sure...until some crazy Mac guys hack the thing and run a Stream benchmark.